Many nuclear receptors act in trans to repress transcriptional responses to signaling pathways as a central aspect of their biological functions, but the underlying mechanisms remain an important and unresolved question in development and homeostasis. Studies performed during the current funding period of this grant demonstrated that the glucocorticoid receptor (GR), the peroxisome proliferator-activated receptor y (PPAR?) and liver X receptors (LXRs) repress responses to toll-like receptor activation in a promoter-, and nuclear receptor-specific manner. The current proposal seeks to define the underlying mechanisms responsible for these specific programs of repression on a genome-wide scale. Preliminary studies indicate that PPAR? represses a subset of inflammatory response genes in macrophages by preventing the signal- dependent dissociation of NCoR co-repressor complexes. Surprisingly, this mechanism involves ligand- dependent sumoylation of the PPAR? ligand binding domain, which targets PPAR? to NCoR/HDACs co- repressor complexes on inflammatory gene promoters. The interaction of sumoylated PPAR? with NCoR/HDAC3 complexes prevents the recruitment of ubiquitylation/igS proteosome machinery that normally mediates their signal-dependent removal. Studies in Drosophila Schneider cells suggests that this sumoylation-dependent transrepression pathway is evolutionarily conserved and is utilized by both orphan and ligand-dependent nuclear receptors. Based on these observations, we propose to explore the roles of sumoylation and NCoR co-repressor complexes in nuclear receptor-dependent transrepression of innate immune responses on a genome-wide scale. Three Specific Aims are proposed. Specific Aim i will test the hypothesis that sumoylation of nuclear receptors is a broadly used mechanism in transrepression of inflammatory programs of gene expression. Specific Aim 2 will test the hypothesis that sumoylation of nuclear receptors mediates repression of inflammatory programs of gene expression by preventing co- repressor/co-activator exchange. Specific Aim 3 will test the hypothesis that the function of NCoR co- repressor complexes is modulated by the histone methyltransferase SMYDs. The results of these studies are likely to lead to new insights into the mechanisms underlying transrepression of inflammatory responses that can be exploited for development of novel therapeutic approaches.